Summary TXNIP is an α-arrestin family protein that is induced in response to glucose elevation. It has been shown to provide a negative feedback loop to regulate glucose uptake into cells, though the biochemical mechanism of action has been obscure. Here, we report that TXNIP suppresses glucose uptake directly by binding to the glucose transporter, Glut1, inducing Glut1 internalization through clathrin coated pits, as well as indirectly by reducing the level of Glut1 mRNA. In addition, we show that energy stress results in phosphorylation of TXNIP by AMP-dependent protein kinase (AMPK), leading to its rapid degradation. This suppression of TXNIP results in an acute increase in Glut1 function and an increase in Glut1 mRNA (hence total protein levels) for long-term adaptation. The glucose influx through GLUT1 restores ATP/ADP ratios in the short run and ultimately induces TXNIP protein production to suppress glucose uptake once energy homeostasis is reestablished.

Brk (for breast tumor kinase) is a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. Brk was originally identified from a human metastatic breast tumor, and its overexpression is frequently observed in breast cancer and several other cancer types. However, the molecular mechanism by which this kinase participates in tumorigenesis remains poorly characterized. In the present study, we not only identified paxillin as the binding partner and substrate of Brk but also discovered a novel signaling pathway by which Brk mediates epidermal growth factor (EGF)-induced paxillin phosphorylation. We show that EGF stimulation activates the catalytic activity of Brk, which in turn phosphorylates paxillin at Y31 and Y118. These phosphorylation events promote the activation of small GTPase Rac1 via the function of CrkII. Through this pathway, Brk is capable of promoting cell motility and invasion and functions as a mediator of EGF-induced migration and invasion. In accordance with these functional roles, Brk translocates to membrane ruffles, where it colocalizes with paxillin during cell migration. Together, our findings identify novel signaling and biological roles of Brk and indicate the first potential link between Brk and metastatic malignancy.Unraveling the signaling pathways responsible for the establishment of a metastatic phenotype in carcinoma cells is of crucial importance for the understanding of the pathology of cancer. The process of metastasis includes several components, such as the ability to invade through acquisition of cell motility, degradation of extracellular matrix and basement membrane, cell proliferation, and survival signaling. Aberrant tyrosine kinase signaling via stimulation of growth factor receptors or intracellular tyrosine kinases has been shown to contribute to various steps of tumor development and progression, including metastasis (6). Brk is an intracellular tyrosine kinase that was identified in a study for screening kinases expressed in human metastatic breast tumors (36). In addition to a typical tyrosine kinase domain, Brk possesses both SH3 and SH2 domains and thus is related to Src family kinases (36). However, unlike Src family kinases, Brk lacks an N-terminal consensus sequence for myristoylation and membrane association (36). Its genomic structure is also distinct from that of Src family kinases, suggesting that Brk has diverged significantly from Src kinases in evolution (37). The expression pattern and subcellular localization of Brk have suggested its role in tumorigenesis. In normal tissues, the expression of Brk or its mouse ortholog Sik is restricted to differentiating epithelial cells of the skin and gastrointestinal tract (27). However, it is highly expressed in many breast carcinoma cell lines and a significant portion of breast tumor tissues but not in human mammary epithelial cells (3, 34, 36) and mouse mammary glands at various developmental stages (27). Elevated expression of Brk has also been detected in metastatic melanoma cell li...

Summary BRAF is an oncogenic protein kinase that drives cell growth and proliferation through the MEK-ERK signaling pathway. BRAF inhibitors have demonstrated anti-tumor efficacy in melanoma therapy, but have also found to be associated with the development of cutaneous squamous cell carcinomas (cSCC) in certain patients. Here, we report that BRAF is phosphorylated at Ser729 by AMP-activated protein kinase (AMPK), a critical energy sensor. This phosphorylation promotes the association of BRAF with 14-3-3 proteins and disrupts its interaction with the KSR1 scaffolding protein, leading to attenuation of the MEK-ERK signaling. We also show that phosphorylation of BRAF by AMPK impairs keratinocyte cell proliferation and cell cycle progression. Furthermore, AMPK activation attenuates BRAF inhibitor-induced ERK hyperactivation in keratinocytes and epidermal hyperplasia in mouse skin. Our findings reveal a mechanism for regulating BRAF signaling in response to energy stress and suggest a strategy for preventing the development of cSCC associated with BRAF-targeted therapy.